For the purposes of Federal and Massachusetts income tax returns you are required to file under the status of either "joint" or "married, filing separately" if you are married to your spouse on December 31st of the tax year in question. Therefore, if you are in the process of getting a divorce but were still legally married on December 31, 2010, then you must file under one of these two statuses for the year 2010.
There is no specific law that requires you to file under the status of "joint" or "married filing separately", and typically this is your choice. A Judge can order parties to file jointly if there will be a benefit (such as a higher refund), but this is not typical.
In each case the choice of how to file can be affected by different factors. For example, if you are concerned that your spouse is hiding income, then you definitely would not want to file a "joint" tax return because you could be liable for his/her hidden income.
In most cases it makes sense to file jointly because their is usually a financial benefit, but in your case you should consult with an accountant to figure out the best option for you.